Industrial processing oils such as cutting and grinding oils are essential to the metal processing field since they serve for the lubrication of sliding surfaces of tools, the cooling of tools and the materials to be cut and/or ground, the cleaning or removal of sludge, chips and the like, and thus are consumed in large quantities in that field. Among them, emulsion-type processing oils wherein the oils are emulsified with, for example, a surfactant have increasingly been used, because of the increased need of the improvement of the working environment, the reduction of fire risk required for the employment of unmanned system, and the like. Such emulsion-type processing oils have the following general composition.
TABLE 1 ______________________________________ Component Amount (w/v %) ______________________________________ Mineral oil 50-80 Fat, fatty acid 0-50 High-pressure additive 0-50 Surfactant 15-35 Alkanolamine, alkali 0-5 Polyol, glycol 0-10 Organic inhibitor 0-5 Rut-proof microbicide &lt;2 Anti-corrosive agent &lt;0.5 for copper alloy Anti-foaming agent &lt;0.5 Water 0-10 ______________________________________ Note: this processing oil is diluted ten to fiftyfold with water before use.
Such emulsion-type processing oils are useful in terms of their ability to reduce the fire risk, as described above. However, since the processing oils are used after they are diluted with water, it is inevitable to cause the propagation of aerobic or anaerobic bacteria, fungi and yeasts in the processing oils. More specifically, these microorganisms will propagate in the storage tanks and pipes through which the processing oils are circulated (reportedly, the concentration usually reaches from several ten millions to several hundred millions per ml) and, as a result, an offensive odor develops from their metabolites such as ammonia, methylamine, hydrogen sulfide, lower hydrocarbons, and volatile fatty acids. Thus, the pollution of the working environment by this odor has become a new problem. Further problem arises from the formation of the organic acids, such as lactic acid, which then lowers the pH of the emulsion and thus may result in the separation of the emulsion into oil and water (i.e., the loss of emulsion stability) and the metal corrosion.
In order to solve these problems, there have been taken the following measures: monitoring the emulsion-type processing oil by periodically measuring, for example, the concentration of the metabolites, pH, and the bacterium number; cleaning the tank and pipe; renewal of the processing oil; addition of an antiseptic agent; removal of fat components with an oil skimmer; making an aerobic condition by air-bubbling; and the like. However, these measures do not drastically solve the above problems. For example, an emulsion-type processing oil is pre-adjusted to a pH of about 9-10 to provide itself with anti-corrosive and anti-bacterial activities and, at such pH, the antiseptic agent which is added is degraded or deteriorated in the processing oil, and thus the desired antiseptic effect cannot be obtained by such agent. Further, in practice, an aerobic environment wherein the liquid continually flows and an anaerobic environment wherein the residue such as sludge and chips is deposited, co-exist in one system, and therefore various normal bacteria adapted to the respective environments, such as lactic acid-producing and sulfate-reducing bacteria, can be present and propagate in the same system. This makes it more difficult to select an effective measure for solving the above problems.
Under these circumstances, the emulsion-type processing oil is required to be completely replaced with fresh oil every three to six months. Since the replacement procedure including the subsequent thermal disposal of the waste oil consumes much material and time, the cost of the replacement procedure accounts for a considerable proportion of the production cost. Therefore, it is highly desirable to prolong the working life of the emulsion-type processing oil by a means which can readily be carried out at a low cost.